Molybdenum is an essential trace element that is found in enzymes involved in purine metabolism, sulfur metabolism, and the assimilation of inorganic nitrogen. Exposure to excess molybdenum has been linked with gout, and molybdenum levels in the soil affect the levels of nitrate and nitrite and also interfere with normal copper metabolism. In humans, fatal simultaneous deficiencies in enzyme activity for xanthine oxidase and sulfite oxidase have been shown to result from an inborn deficiency of the "molybdenum cofactor," an essential constituent of both enzymes. This research is directed toward understanding this common molybdenum cofactor, which is thought to be a monomeric molybdenum center with ligating sulfur atoms. One objective of this research is to synthesize and to structurally, chemically and spectroscopically characterize molybdenum complexes which are models for the molybdenum centers of these enzymes. Special emphasis will be given to complexes containing a monomeric oxo-molybdenum center linked to an iron porphyrin center as a model for sulfite oxidase. The compounds and their reactions with substrates and inhibitors of molybdoenzymes win be studied by electron paramagnetic resonance spectroscopy (EPR), spectroelectrochemistry, electronic spectroscopy and x-ray crystallography. A second objective is to directly investigate the molybdenum center of the enzyme sulfite oxidase by studying electron transfer processes between the molybdenum and iron centers of the enzyme and by continuing to grow crystals of sulfite oxidase with the ultimate goal of determining the three dimensional structure of the enzyme.